1. Field of the Invention
The present invention relates to anti-lock brake control systems for vehicles having tandem axles and, more particularly, to such a system wherein incipient or actual lockup is sensed on the forward or more lightly loaded axle of the tandem axle pair and the brake pressure and/or rate of increase in brake pressure on the rear or more heavily loaded axle is thereafter reduced or controlled.
2. Description of the Prior Art
Anti-lock brake control systems and devices are well known in the prior art and are exemplified by U.S. Pat. Nos. 3,767,270, 3,768,872, 3,893,696, 3,929,382, 3,929,383, 3,966,267, 4,591,213; 4,724,935 and 4,130,323, the disclosures of all of which are hereby incorporated by reference. Under these systems, incipient or actual skid or wheel lockup is sensed, brake pressure is relieved, and the wheel or wheels are permitted to regain rotational motion prior to reapplication of brake pressure. Maximum vehicle stability together with maximum braking force is obtained by maintaining the longitudinal slip of the braked wheels within predetermined limits. Sensing devices to sense incipient or actual skid or wheel lockup are also well known in the prior art and are exemplified by U.S. Pat. Nos. 3,769,533; 3,854,556; 3,961,214 and 3,980,913, the disclosures of all of which are hereby incorporated by reference.
Anti-lock brake control systems for tandem axles under the prior art typically utilize a sensor for each wheel-end of one or more of the tandem axles, and a brake relief valve for each wheel-end or axle or for all axles; or (2) utilized a sensor on each axle to sense average wheel-end speed and a brake relief valve for that axle or for all axles.
The problem with these systems was their complexity and lack of cost-effectiveness and efficiency. By utilizing multiple sensors and/or brake relief valves, the initial cost of the system was increased, the cost of installation and maintenance was increased, and the likelihood of breakdown was enhanced. Furthermore, these systems failed to make use of the fact that, with most tandem axle suspension systems, the forward axle of the tandem axle pair is more lightly loaded during braking and accordingly will lockup before the rear axle, giving an early warning of incipient lockup of the rear axle. In conjunction with this fact is the fact that the rear axle is more heavily loaded during braking than the forward axle and therefore is more important to control than the forward axle of the tandem axle pair. These same loading conditions are also present when the tandem axle is equipped with a weight transfer system which transfers load from the rear axle to the front axle when the brakes are applied.